The invention is directed to a backing strip for welding technology in one-sided welding of steel, comprising a flexible thread structure of inorganic fibers having a more easily meltable forming layer and a less easily meltable base layer.
Traditional double-sided welding of steel requires that subsequent welding is performed on the welding seam root and that the welding seam root is to be cleaned and planed from the front side. These procedures are time-consuming and bothersome and in some cases even impossible, e.g. in the welding of tubes, containers and elements in ship construction. In such special cases, welding is carried out from one side only. In butt welding, notably, a backing structure is used for root formation, i.e. for the formation of the welding bead on the backside of the seam. This backing structure must be adapted to limit the height and the width of the root and to form such an angle at the transition to the surface of the welded object that the weld joint has high strength and concentration of tension is avoided.
If the surfaces to be welded are linear or are only slightly curved, no particular demands are posed to the flexibility of the backing structure. In one-sided butt welding of pipes and similar strongly curved surfaces, however, there are needed flexible backing structures provided as strips of inorganic fibers. Such flexible backing structures are described in JP 47-94339 and JP-53-45297. These known backing structures consist of packed glass fiber strips being either bonded or pressed together and accommodated in a flexible cover. Welding involves the risk that the bonding material burns out or the cover is destroyed so that the backing strip, because of its disintegration, can be detached only with difficulties from the welding seam. These strips are adapted for one-time use only.
A flexible backing strip of the above type is disclosed in the Soviet author certificate No. 093 010. This known backing strip comprises a more easily meltable forming layer of glass fiber fabric and a less easily meltable base layer of glass fiber fabric, with the strengths of the fibers being identical. The different melting temperatures are obtained by the composition of the respective glass fibers which is adapted to the one-sided welding of unalloyed steel and steel with low carbon content. The base layer and the forming layer are produced as separate band-shaped strips which during manufacture of the backing strip are connected by mutual adhesion. Therefore, it cannot be excluded that the forming layer and the base layer become separated during welding so that multiple use is mostly impossible. The backing strip is bonded by an adhesive layer onto a wider flexible carrier strip enabling adhesion of the backing strip on the object to be welded. It is also known to provide the flexible carrier strip with permanent magnets for effecting attachment on the metal surface of the object to be welded. This type of backing strip suffers from a restricted range of applications, from being limited to one-time use and from decreased effectiveness when welding in non-horizontal positions.